Support automatic execution of scripts on VirtualBox shared folders

If an executable /media/sf_automation/$distri is present (where
$distri defaults to "grml" if unset) it is automatically executed
during bootup. The '/media/sf_automation' directory corresponds
to a shared folder named "automation" inside VirtualBox (which
has the auto-mount option set inside the VirtualBox configuration
to be useful).

The feature can be disabled via boot option 'novbautomation' and
the path to the automation directory can be controlled via boot
option 'vbautomation' (e.g. 'vbautomation='/media/sf_foobar', so
don't forget the "/media/sf_").

This feature provides a nice way to automate things during bootup
inside VirtualBox without having to adjust any boot options,
while still being able to control the behaviour from the host
system via setup of shared folders.